If I were to want to carry heavy loads, I believe I'd have to go with a trailer. CG remains low, the cargo weight is supported on another one or two wheels (depending on the trailer design), and if you don't really need the capacity for the errand that you are currently on, you can leave the trailer at home and not have to lug around the extra weight.
Plus there are lot's of trailers out there designed to haul kids around that can be picked up for yard-sale prices and easily converted to cargo duty.